U.S. Pat. No. 5,460,329 to Sturman describes a hydraulically-actuated fuel injector that is controlled in its operation with a dual solenoid control valve. Each injection event is initiated by energizing one of the solenoids. This pulls a spool valve member from one position to another position, which suddenly exposes an intensifier piston to a high pressure actuation fluid inlet. The piston begins a downward stroke from the force provided by the high pressure actuation fluid. The piston in turn pushes a plunger that pressurizes fuel within the injector body. When the fuel pressure reaches a valve opening pressure sufficient to overcome a compression biasing spring, the needle valve member lifts to open the nozzle outlet to commence the injection of fuel.
Each injection event is ended by energizing the second solenoid to pull the spool valve member back to its original position. This movement of the spool valve member ends the exposure of the intensifier piston to the high pressure actuation fluid inlet, and then exposes the piston to a low pressure drain. Without the high pressure force acting on the piston, the piston and plunger cease their downward movement, and fuel pressure under the plunger drops. Eventually, fuel pressure drops below a valve closing pressure that is sufficient to hold the needle valve open, and the needle valve member is then pushed toward its closed position by its compression biasing spring.
Those skilled in the art have long known that, as a general rule, combustion efficiency is improved and undesirable exhaust emissions are reduced when the injection event is ended as abruptly as possible. Several factors contribute to determining how abrupt an injection event ends. Among these are how fast fuel pressure drops, the area of the lifting hydraulic surface exposed to the fuel pressure, the magnitude of the force tending to push the needle valve member toward its closed position and the mass properties of the needle valve member itself.
The present invention is directed to hastening the closure rate of needle valve members in hydraulically-actuated fuel injectors.